1. Field of the Invention
The present invention relates to a method for operating a semiconductor device of lateral double diffused metal oxide semiconductor (LDMOS). In particular, the present invention is directed to a method for operating a semiconductor device of lateral double diffused metal oxide semiconductor (LDMOS) integrated with a junction field effect transistor (JFET). Such semiconductor device may be selectively turned on and turned off in accordance with a predetermined design.
2. Description of the Prior Art
With the coming of the mobile communication era, all kinds of wireless communication devices, such as mobile phones, global positioning systems (GPS), or cell phone base stations, are growing rapidly in recent years, A high voltage device such as the lateral double diffused metal oxide semiconductor (LDMOS) are often designed to be the control circuit of the radio frequency (900 MHz-2.4 GHz) because it not only has high operating bandwidth but also has higher break down voltage and higher output power due to element design and manufacturing process, so it is widely used in the power amplifier of all kinds of wireless communicating systems.
FIG. 1 illustrates a cross section view of a conventional lateral double diffused metal oxide semiconductor. The lateral double diffused metal oxide semiconductor 110 has a gate 111, a source 112, and a drain 113. When the threshold voltage of the lateral double diffused metal oxide semiconductor needs adjusting, another reticle is needed and an ion implantation should be carried out to implant different dopant. It is well known that either amending an old reticle or constructing a new reticle all cost a lot of money. Accordingly, this additional reticle is a heavy cost burden to the manufacturer of the lateral double diffused metal oxide semiconductor.
Second, because the lateral double diffused metal oxide semiconductor is a semiconductor device of depletion mode, the concentration of the dopant in the substrate need controlling to adjust the threshold voltage of the lateral double diffused metal oxide semiconductor. That is, the concentration of the dopant in the region 1 needs adjusting to adjust the threshold voltage of the lateral double diffused metal oxide semiconductor 110. However, if the threshold voltage should be higher, the concentration of the dopant should be higher, too. Obviously, the higher the concentration of the dopant, the lower the electrical resistance of the substrate is so the region 1 is always conductive.
Since the region 1 is always conductive, it leaves the lateral double diffused metal oxide semiconductor always on (a turn-on state), too and makes the lateral double diffused metal oxide semiconductor hard turn off. Therefore, the dilemma is to have a higher threshold voltage and to turn off the lateral double diffused metal oxide semiconductor easily at the same time. Accordingly, a novel lateral double diffused metal oxide semiconductor is needed. On one hand, there is no need of additional reticle or of increase of dopant concentration to adjust the threshold voltage. On the other hand, the lateral double diffused metal oxide semiconductor may selectively turn on and turn off to be an easily-controlled semiconductor device.